Thermal dissipation probe ( TDP) was used to continuously measure the sap flux density
( Fd
) of Pinus tabuliformis and Hippophae rhamnoides individuals in hilly Loess Plateau，from June
to October 2015，and the environmental factors，i．e．，photosynthetic active radiation ( PAＲ) ，water
vapor pressure deficit ( VPD) ，and soil water content ( SWC) ，were simultaneously monitored to
clarify the difference of rainfall utilization between the two tree species in a mixed plantation． Using
the methods of a Threshold－delay model，stepwise multiple regression analyses，and partial correlation
analyses，this paper studied the process of Fd in these two species in response to the rainfall
pulses and then determined the effects of environmental factors on Fd ． The results showed that，with
the increase of rainfall，the response percentages of Fd in both P． tabuliformis and H． rhamnoides increased
at first but then decreased; specifically，in the range of 0－1 mm rainfall，the Fd of P． tabuliformis
( －16．3%) and H． rhamnoides ( －6．3%) clearly decreased; in the range of 1－5 mm rain-fall，the Fd of P． tabuliformis decreased ( －0．4%) ，whereas that of H． rhamnoides significantly increased
( 9．0%) ． The lower rainfall thresholds ( ＲL ) of Fd for P． tabuliformis and H． rhamnoides
were 6．4 and 1．9 mm，respectively，with a corresponding time-lag ( τ) of 1．96 and 1．67 days． In
the pre － rainfall period，the peak time of Fd of P． tabuliformis converged upon 12: 00－12: 30
( 70%) ，while the Fd of H． rhamnoides peaked twice，between 10: 30 and 12: 00 ( 48%) and again
between 16: 00 and 16: 30 ( 30%) ． In the post－rainfall period，the peak time of Fd of P． tabuliformis
converged upon 11: 00－13: 00 ( 40%) ，while that of H． rhamnoides peaked twice，between
12: 00 and 13: 00 ( 52%) and again between 16: 30 and 17: 00 ( 24%) ． Among the environmental
factors，the rank order of factors associated with the Fd of both P． tabuliformis and H． rhamnoides
was PAＲ＞VPD，before rainfall． However，the rank order of factors influencing the Fd of P． tabuliformis
was PAＲ＞VPD＞0－20 cm SWC ( SWC0－20
) ，whereas this order was different for H． rhamnoides:
SWC0－20＞PAＲ ＞VPD，after rainfall． This mixed plantation of P． tabuliformis and H． rhamnoides
trees had a high stability of water utilization．